Is there a daemon which resolves a service name to a port?
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A DNS server resolves a hostname to an IP address. A program can resolve a hostname to an IP address by calling getaddrinfo() which in turn asks a DNS server to do the resolution, if I am correct.
In SysV init, is there some daemon which resolves a service name to a port, just like a DNS server? Does it do that by reading /etc/services? Does getaddrinfo() also invoke the daemon to perform the resolution?
I know that inetd reads /etc/services to decide which sockets to listen to. But inetd doesn't seem to resolve service name to port, does it?
Thanks.
services sysvinit inetd
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
asked Feb 14 at 0:48
TimTim
27.5k78264476
add a comment |
A DNS server resolves a hostname to an IP address. A program can resolve a hostname to an IP address by calling getaddrinfo() which in turn asks a DNS server to do the resolution, if I am correct.
In SysV init, is there some daemon which resolves a service name to a port, just like a DNS server? Does it do that by reading /etc/services? Does getaddrinfo() also invoke the daemon to perform the resolution?
I know that inetd reads /etc/services to decide which sockets to listen to. But inetd doesn't seem to resolve service name to port, does it?
Thanks.
services sysvinit inetd
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
asked Feb 14 at 0:48
TimTim
27.5k78264476
add a comment |
A DNS server resolves a hostname to an IP address. A program can resolve a hostname to an IP address by calling getaddrinfo() which in turn asks a DNS server to do the resolution, if I am correct.
In SysV init, is there some daemon which resolves a service name to a port, just like a DNS server? Does it do that by reading /etc/services? Does getaddrinfo() also invoke the daemon to perform the resolution?
I know that inetd reads /etc/services to decide which sockets to listen to. But inetd doesn't seem to resolve service name to port, does it?
Thanks.
services sysvinit inetd
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
asked Feb 14 at 0:48
TimTim
27.5k78264476
A DNS server resolves a hostname to an IP address. A program can resolve a hostname to an IP address by calling getaddrinfo() which in turn asks a DNS server to do the resolution, if I am correct.
In SysV init, is there some daemon which resolves a service name to a port, just like a DNS server? Does it do that by reading /etc/services? Does getaddrinfo() also invoke the daemon to perform the resolution?
I know that inetd reads /etc/services to decide which sockets to listen to. But inetd doesn't seem to resolve service name to port, does it?
Thanks.
services sysvinit inetd
services sysvinit inetd
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
asked Feb 14 at 0:48
TimTim
27.5k78264476
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
asked Feb 14 at 0:48
TimTim
27.5k78264476
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
edited Feb 17 at 13:43
Jeff Schaller
43.2k1159138
43.2k1159138
asked Feb 14 at 0:48
TimTim
27.5k78264476
asked Feb 14 at 0:48
TimTim
27.5k78264476
asked Feb 14 at 0:48
TimTim
27.5k78264476
27.5k78264476
add a comment |
add a comment |
1 Answer
1
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oldest
votes
Note that getaddrinfo() only calls DNS if nsswitch.conf defines it. The files entry reads, directly, from /etc/hosts.
Basically, the "name service switch" functions (NSS) look at /etc/nsswitch.conf and then may dynamically load "libnss" routines. So, for example, you might have /lib/x86_64-linux-gnu/libnss_files.so.2, which is the library loaded when files is mentioned in nsswitch.conf.
For hosts, the nsswitch.conf line may read something like
hosts: files dns
This will tell the name service resolver to load the "nss_files" library (which will look in /etc/hosts) and if that fails, load the "nss_dns" library. It's that library that calls out to a DNS server.
For services, the nsswitch.conf line may read something like
services: files ldap
This will load the "nss_files" library (which will look in /etc/services), and if that fails then load the "nss_ldap" library.
There are various different backends (files, db, ldap, nis, compat, dns...) and they determine how names are resolved.
For an inet entry such as
service time
...
the time value is looked up in the NSS map for services. If files is used in nsswitch.conf then it will see a line
time 37/tcp timserver
which tells inetd to bind to TCP port 37.
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
add a comment |
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1 Answer
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1 Answer
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oldest
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oldest
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votes
Note that getaddrinfo() only calls DNS if nsswitch.conf defines it. The files entry reads, directly, from /etc/hosts.
Basically, the "name service switch" functions (NSS) look at /etc/nsswitch.conf and then may dynamically load "libnss" routines. So, for example, you might have /lib/x86_64-linux-gnu/libnss_files.so.2, which is the library loaded when files is mentioned in nsswitch.conf.
For hosts, the nsswitch.conf line may read something like
hosts: files dns
This will tell the name service resolver to load the "nss_files" library (which will look in /etc/hosts) and if that fails, load the "nss_dns" library. It's that library that calls out to a DNS server.
For services, the nsswitch.conf line may read something like
services: files ldap
This will load the "nss_files" library (which will look in /etc/services), and if that fails then load the "nss_ldap" library.
There are various different backends (files, db, ldap, nis, compat, dns...) and they determine how names are resolved.
For an inet entry such as
service time
...
the time value is looked up in the NSS map for services. If files is used in nsswitch.conf then it will see a line
time 37/tcp timserver
which tells inetd to bind to TCP port 37.
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
add a comment |
Note that getaddrinfo() only calls DNS if nsswitch.conf defines it. The files entry reads, directly, from /etc/hosts.
Basically, the "name service switch" functions (NSS) look at /etc/nsswitch.conf and then may dynamically load "libnss" routines. So, for example, you might have /lib/x86_64-linux-gnu/libnss_files.so.2, which is the library loaded when files is mentioned in nsswitch.conf.
For hosts, the nsswitch.conf line may read something like
hosts: files dns
This will tell the name service resolver to load the "nss_files" library (which will look in /etc/hosts) and if that fails, load the "nss_dns" library. It's that library that calls out to a DNS server.
For services, the nsswitch.conf line may read something like
services: files ldap
This will load the "nss_files" library (which will look in /etc/services), and if that fails then load the "nss_ldap" library.
There are various different backends (files, db, ldap, nis, compat, dns...) and they determine how names are resolved.
For an inet entry such as
service time
...
the time value is looked up in the NSS map for services. If files is used in nsswitch.conf then it will see a line
time 37/tcp timserver
which tells inetd to bind to TCP port 37.
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
add a comment |
Note that getaddrinfo() only calls DNS if nsswitch.conf defines it. The files entry reads, directly, from /etc/hosts.
Basically, the "name service switch" functions (NSS) look at /etc/nsswitch.conf and then may dynamically load "libnss" routines. So, for example, you might have /lib/x86_64-linux-gnu/libnss_files.so.2, which is the library loaded when files is mentioned in nsswitch.conf.
For hosts, the nsswitch.conf line may read something like
hosts: files dns
This will tell the name service resolver to load the "nss_files" library (which will look in /etc/hosts) and if that fails, load the "nss_dns" library. It's that library that calls out to a DNS server.
For services, the nsswitch.conf line may read something like
services: files ldap
This will load the "nss_files" library (which will look in /etc/services), and if that fails then load the "nss_ldap" library.
There are various different backends (files, db, ldap, nis, compat, dns...) and they determine how names are resolved.
For an inet entry such as
service time
...
the time value is looked up in the NSS map for services. If files is used in nsswitch.conf then it will see a line
time 37/tcp timserver
which tells inetd to bind to TCP port 37.
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
Note that getaddrinfo() only calls DNS if nsswitch.conf defines it. The files entry reads, directly, from /etc/hosts.
Basically, the "name service switch" functions (NSS) look at /etc/nsswitch.conf and then may dynamically load "libnss" routines. So, for example, you might have /lib/x86_64-linux-gnu/libnss_files.so.2, which is the library loaded when files is mentioned in nsswitch.conf.
For hosts, the nsswitch.conf line may read something like
hosts: files dns
This will tell the name service resolver to load the "nss_files" library (which will look in /etc/hosts) and if that fails, load the "nss_dns" library. It's that library that calls out to a DNS server.
For services, the nsswitch.conf line may read something like
services: files ldap
This will load the "nss_files" library (which will look in /etc/services), and if that fails then load the "nss_ldap" library.
There are various different backends (files, db, ldap, nis, compat, dns...) and they determine how names are resolved.
For an inet entry such as
service time
...
the time value is looked up in the NSS map for services. If files is used in nsswitch.conf then it will see a line
time 37/tcp timserver
which tells inetd to bind to TCP port 37.
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
answered Feb 14 at 1:23
Stephen HarrisStephen Harris
26.5k34779
26.5k34779
add a comment |
add a comment |
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